I'm running vista 64 and having sound issues with this game. I get background effects but no dialogue or main audio. The same issues are being raised on AC forums, but so far none of the advice works for me. Ubisoft haven't replied to my support email and there are no patches yet for the PC version.
Advice so far, (1) turn down the acceleration on the sound device settings (Can't do in vista, but seems to work for everyone else). (2) replace a dll file - eax.dll (again, doesn't work in vista as the file isn't required in the first place in vista)
Think I'll go play something else, until Ubisoft can be bothered to supply a patch.